This invention relates to a control device for a multi-stage automatic transmission with three forward gear ratios, with speed changing under torque smoothed out by a hydrokinetic torque converter, and one reverse drive ratio.
Automatic transmissions of this type are already well known for automotive vehicles, and they consist mainly of a hydrokinetic torque converter and epicyclic train gearbox combined with a set of hydraulic and sometimes electronic control systems which take into consideration, in controlling the various components of the gearbox, data on the motion of the vehicle such as, in particular, its speed, as well as the motor load or, which amounts to the same thing, the position of the accelerator pedal.
The combination of a torque converter and an epicyclic gear train makes it possible to get a continuous variation in the torque applied to the vehicle wheels when starting or accelerating. In addition, it is possible to obtain gear changes which can be rapidly carried out under torque and are smoothed out by the hydrokinetic torque converter, thus providing the transmission with great flexibility in use.
The epicyclic gear train is piloted by a number of hydraulic receivers. In the case of a transmission with three forward ratios and one reverse it is thus possible to use two clutches and two brakes controlled hydraulically. The change from one ratio to the other is performed by locking or releasing one of these receivers and by releasing or locking the other receiver. It is necessary to provide a transient period, in particular when changing from the third to the second forward ratio and vice versa in order to enable the vehicle drive motor to adapt its speed to the new transmission ratio.
These clutches and brakes are controlled through a hydraulic unit monitored by two electric valves in order to feed the two clutches and the two brakes selectively, so that the different ratios of the transmission are obtained. The selective feed of the said electric valves takes into account the information on the running of the vehicle equipped with the transmission. In practice, an electronic device called a governor-comparator supplies signals corresponding to the threshold of the change from the first to the second forward drive ratio and to the threshold of the change from the second to the third forward drive ratio. In addition, this electronic device supplies a signal which depends upon the position of the accelerator pedal, thus providing information on the load of the vehicle drive motor. Finally, the device also supplies a signal corresponding to the vehicle speed.
The electronic governor-comparator takes these different pieces of information into account as well as the position of the selector lever handled by the driver and a so-called "kick-down" signal, which is placed at the end of the accelerator travel and enables the changing down of the ratios to be forced within a certain allowable range of speeds, in order to supply the two electric valves controlling the hydraulic circuit which monitors the gearbox with signals capable of opening or closing these electric valves.
French Pat. No. 1,528,878 describes a hydraulic control device for such an automatic transmission. In that patent, the epicyclic gear train is operated by means of two clutches and two brakes which are hydraulically controlled, while the epicyclic train also includes a freewheel system. The hydraulic control system comprises, mainly, a hydraulic selector or control valve which can take up three positions corresponding to the three forward speeds and which is monitored by two electric valves in order to feed selectively the two clutches and two brakes mentioned above, in order to define the three forward ratios of the transmission. A hydralic valve, called a relay valve, is also inserted in the hydraulic circuit between the hydraulic selector valve and the receivers formed by the above-mentioned clutches and brakes. This relay valve makes it possible to produce the overlap necessary between the second and third speeds to obtain a third-to-second change or vice versa without any break in the mechanical connection between the gearbox input and the output shafts, without racing the motor and without jerkiness for the vehicle driver. This overlap time is defined by hydraulic means.
French Pat. No. 2,313,608 describes another embodiment applying hydraulic delay means acting on change from the third to the second ratio and vice versa.
These hydraulic delays, which depend either on the filling time of one of the receivers or on the time of movement of the hydraulic valve spools can only, however, be determined approximately, without taking into consideration all the parameters concerning the running of the vehicle.
One object of this invention is to provide a device to control such an automatic transmission which accomplishes a smooth changeover between the third and second forward speed ratios.
Another object of the present invention is to make it possible to separate the function of control of the various components of the automatic transmission from the delay function required to determine the duration of the transient state between these two transmission speed ratios. It then becomes possible to entrust the delay function to means specially designed for this purpose, preferably electonic means in particular.
Another object of this invention is to provide the two electric valves monitoring the hydraulic control device for such a transmission with signals, particularly for the change from the third to the second forward drive ratio and vice versa, which makes it possible to obtain a suitable duration of the transient state of the control device which corresponds to the fourth state of the two electric valves.